1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to an apparatus and method for performing handoff in a heterogeneous communication system in which different communication schemes coexist.
2. Description of the Related Art
In the 4th generation (4G) communication system, extensive research is being conducted to provide high-speed services having various Qualities of Service (QoS) to users. Particularly, extensive research is being conducted to support high-speed services capable of guaranteeing mobility and QoS for a Broadband Wireless Access (BWA) system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
In addition, the current communication systems, i.e. 3rd generation (3G) communication system and 3.5th (3.5G) communication system, are evolving, proposing new communication schemes. The proposed communication schemes include High Speed Downlink Packet Access (HSDPA), Evolution-Data Only (EV-DO), Evolution-Data and Voice (EV-DV), Institute of Electrical and Electronics Engineers (IEEE) 802.16, IEEE 802.11, and Digital Multimedia Broadcasting (DMB).
Therefore, the communication systems now under development may adopt the foregoing advanced communication standards, or may employ new innovative technologies replacing the above communication schemes. However, due to the advent of the various communication schemes mentioned above, the next generation communication system will evolve into a heterogeneous communication system in which various communication schemes coexist.
The communication system should take mobility of Mobile Stations (MSs) into consideration, and provide handoff so that the MS may maintain a call no matter where the MS moves.
FIG. 1 is a graph illustrating intercell pilot signal strength for handoff in a conventional communication system.
Referring to FIG. 1, a conventional handoff will be described for an Interim Standard-95 (IS-95) (or cdmaOne) communication system using Code Division Multiple Access (CDMA), by way of example.
A Base Station (BS) currently providing a service to a corresponding MS, i.e. a serving BS, will be called a BS A. The MS measures currently received pilot signals, and the measured pilot signals are pilot signals of the BS A, a BS B and a BS C. After measuring the pilot signal strengths, the MS periodically reports the measured pilot signal strengths to the BS A.
The BS A determines whether to perform handoff of the MS, depending on the pilot signal strengths received from the MS. The pilot signal strengths can be expressed in dBm or dB, and indicate Received Signal Strength (RSS) or Signal to Interference and Noise Ratio (SINR), which reflects the strength of the radio signals.
The graph of FIG. 1 shows strengths of pilot signals that the MS receives from a plurality of BSs, i.e. the BS A, the BS B and the BS C, with the passage of time. In the graph, the vertical axis represents signal the pilot signal strengths, and the horizontal axis represents time.
In the IS-95 communication system, the MS classifies BSs into an aggregate according to strengths of the received pilot signals, and manages the aggregate. The aggregate managed in the MS is called a ‘set’.
The set is roughly divided into two types, and the MS classifies the BSs into a candidate set and an active set. Of the BSs, a BS whose pilot signal strength is greater than a first threshold T_ADD is classified as an active set, and a BS whose pilot signal strength is less than a second threshold T_DROP is excepted as the active set.
It will be assumed in FIG. 1 that the MS now in communication with the BS A leaves the service coverage area of the BS A and moves to service coverage area of the BS B or the BS C, attempting handoff. The above handoff method is called soft handoff, and the number of active sets of the MS is limited to, for example, is 2.
While the pilot signal strength of the BS A measured by the MS decreases with time, the pilot signal strengths of the BS B and the BS C increase slightly with time.
A handoff operation of the MS will now be described along with the passage of time. First, at time ‘a’, the MS, since it is communicating with the BS A, classifies the BS A as a serving BS, and at the same time, classifies the BS A as an active set. At this moment, the pilot signal strengths of the BS B and the BS C are both less than the first threshold T_ADD. Therefore, the MS classifies the BS B and the BS C as a candidate set.
Second, at the time ‘a’, the pilot signal strength of the BS B is greater than the first threshold T_ADD. Therefore, the MS classifies the BS B as an active set, and the MS communicates not only with the BS A but also with the BS B. In addition, the MS classifies the BS C as a candidate set.
Third, at time ‘b’, the pilot signal strength of the BS C is greater than the first threshold T_ADD. However, because the number of active sets is limited to 2, the MS compares the pilot signal strengths of the BS B and the BS C. At this time, the pilot signal strength of the BS C is less than the pilot signal strength of the BS B. Therefore, the BS C is classified as a candidate set, even though it can be classified as an active set.
Fourth, at time ‘c’, the MS compares the pilot signal strength of the BS A with the pilot signal strength of the BS B, and outputs a comparison result T_COMPAB. If the comparison result T_COMPAB is continuously greater than a predetermined threshold T_COMP for a predetermined set time, the MS changes the BS B as a new serving BS.
Fifth, at time ‘d’, the MS compares the pilot signal strength of the BS A and the pilot signal strength of the BS C, and outputs a comparison result T_COMPAC. If the comparison result T_COMPAC is continuously greater than the threshold T_COMP for a predetermined set time, the MS changes the BS C as an active set. Therefore, the MS classifies the BS A as a candidate set. At this moment, the MS stops communication with the BS A, and starts communication with the BS C. If the number of active sets is greater than or equal to 3, the MS can communicate with all of the BS A, the BS B, and the BS C.
Sixth, at time ‘e’, if the BS A is an active set, because the pilot signal strength of the BS A is less than the second threshold T_DROP, the MS classifies the BS A as a candidate set.
Hard handoff is similar to the soft handoff in operation. In the hard handoff though, the BS simply performs a preparation operation for handoff instead of becoming an active set, and at same time, communicating with the MS. As a result, in the hard handoff, inter-BS communication of the MS is performed through one BS.
However, the current communication system, as described above, will evolve into the heterogeneous communication system in which at least two different communication schemes coexist, rather than single (one) communication system using one communication scheme. Even in the heterogeneous communication system, the mobility of MSs should be taken into consideration. Further, handoff for providing seamless communication service should be taken into account due to the movement of the MSs.
FIG. 2 is a diagram illustrating pilot signal strengths received at an MS from BSs using different communication schemes.
Referring to FIG. 2, service coverage of a BS A and service coverage of a BS B, i.e. a cell A 200 and a cell B 250, are shown. Herein, the BS A and the BS are communication systems using different communication schemes.
Assuming that an MS 201 supports both the communication schemes used in the BS A and the BS B, there is a possible scenario in which the MS 201 located in the service coverage area of the BS A moves to the coverage area of the BS B, requiring it to change its serving BS.
The BS A and the BS B use different communication schemes. Therefore, if the MS 201 measures the pilot signal strengths in the existing method of FIG. 1, strengths of the pilot signals transmitted by the BSs may not be consistent due to the discrepancy of the communication schemes.
That is, when the MS performs handoff between different communication systems as shown in FIG. 2, if the minimum value of the pilot signal strength for communication with the BS A is defined as ‘min a’, and the minimum value of the pilot signal strength for communication with the BS B is defined as ‘min b’, the MS cannot use a threshold of an absolute pilot signal strength for classifying BSs, due to a difference between the two minimum values. In addition, when the pilot signal strengths of the two BSs both exceed the minimum values ‘min a’ and ‘min b’ for communication, at the boundary of the cell A and the cell B, the MS cannot determine which BS's signal is preferable for handoff decision. That is, the MS cannot determine the difference between the pilot signal strengths in the method of FIG. 1, to perform handoff. It is also impossible to use such handoff parameters as the first threshold T_ADD, the second threshold T_DROP, and the reference value T_COMP of FIG. 1.
In conclusion, the conventional technology cannot support appropriate handoff due to the movement of an MS between heterogeneous communication system. Accordingly, there is a need for a handoff method for providing a seamless communication service to an MS in a heterogeneous communication system.